1. Field of the Invention
The present invention relates to an image forming apparatus, a control method therefor, and a computer program.
2. Description of the Related Art
Various types of light deflectors which are swing bodies making mirrors resonate have been proposed.
A resonant swing body is characterized by allowing a great reduction in size, having low power consumption, being theoretically free from plane tilt, and the like as compared with an optical system designed to perform optical scanning by using a rotating polygon mirror such as a polygon mirror. A swing body comprising an Si single crystal manufactured by a semiconductor process, in particular, is characterized by being theoretically free from fatigue of metals and excellent in durability (see Japanese Patent Laid-Open No. 57-8520).
In the field of image forming apparatuses using electrophotography, reductions in size and cost are required. In order to meet this requirement, there has been proposed a method using a galvanometer mirror which is a resonant swing body manufactured by a semiconductor process in place of a polygon mirror which has been conventionally used (see Japanese Patent Laid-Open No. 7-175005). According to this method, a galvanometer mirror is made to resonate at a resonance frequency unique to the size of the mirror to scan a light beam in the main-scan direction, thereby forming an image. This galvanometer mirror can be reduced in size by using a semiconductor process, and many mirrors can be manufactured at once. Therefore, a reduction in cost can be expected.
A nested mirror (see Japanese Patent Laid-Open No. 2005-208578) has the property of achieving an almost constant velocity in the scanning range to be used and the property of allowing a large scanning angle to be set. This can form a correction optical system with a simple arrangement, which is suitable for a scanning apparatus in a compact, low-cost image forming apparatus.
When a light beam is deflected by making a swing mirror resonate using the above technique, resonance fluctuations are caused by air turbulence due to the air resistance to the swing mirror at the time of resonance. This may cause nonperiodic jitter.
This jitter appears as the angular velocity jitter of the swing mirror and pixel formation position jitter in the main-scan direction, resulting in width variations in the main-scan direction, as indicated by a transfer medium 101 in FIG. 1. As a consequence, straight lines wobble at the center and end portions on a transfer medium, resulting in deterioration in image quality.